If you don't the salts to premix your LFS also has it mixed already for $0.50 as well. I also have another 5g for just RO/DI water for topping off. I only have a small 29g cube so for now & this will do. If you have a large aquarium then buying the unit is your best bet.

Yeah its always good to have some spare RODI water laying around for topping off and what not i keep 5 gallons at all times just incase i will need it